Slugs

In a typical year, slugs are stimulated out of their summer aestivation following autumn rains and will generally be problematic in paddocks where they have previously been an issue. With the recent rainfall across many regions, slugs will become more active over the coming weeks. Agronomy consultant, Ben Dumesny (MS&A), has reported slug activity in a newly emerging canola paddock near Camperdown, in the Western district of Victoria. The paddock experienced issues with slugs in 2012. All stubble was burnt prior to sowing and baits were applied prior to crop emergence. Thus far, there has been no noticeable feeding damage, although the paddock will be monitored closely. Agronomist, Greg Condon (Grassroots Agronomy), has also reported signs of slug activity in a canola paddock south of Wagga Wagga, in the South West Slopes district of New South Wales. This crop was sown into a cereal stubble.

The main pest species of slugs attacking broad-acre crops are the black keeled slug (Milax gagates) and the grey field slug (Deroceras reticulatum). The brown field slug (Deroceras invadens) can also be a serious pest, with species often co-occurring. Soils that retain moisture are preferred by slugs, typically heavy red loams, gravelly loams and grey clays. 

The most common control method for slugs is to use baits, which are generally applied at sowing after good germinating rains to coincide with slug emergence. Researcher, Michael Nash (The University of Melbourne), says field trails conducted in 2011-2012 demonstrate that ingestion of active toxicants is more effective against slugs than spray applications. When baiting, it is important to consider the conditions, number of baits used and how this relates to slug densities. Baiting efficacy can be reduced when there are higher slug populations, an increased availability of refuges, and higher amounts of plant material.

Economic thresholds for slugs vary depending on the species and field conditions, hence are only a rough guide. For the grey field slug, suggested thresholds are 1 slug per m² in canola and pulses, and 5 per m² in cereals and pastures, whereas for the black-keeled slug, suggested thresholds are less than 1 per m² in canola, cereals and pulses and 5 per m² in pastures.

Using terracotta tiles, or another type of refuge ‘trap’ is the best way to assess numbers. Traps should be placed on the soil surface when it is visibly wet, and then checked after a few days for the presence of slugs underneath. A few pieces of slug bait under each trap will help to attract slugs if they are present. Other management options that should be considered are burning stubbles to remove refuges, and removing green material from paddocks prior to sowing. Rolling after sowing to consolidate the seedbed provides some control, and the use of shallow discs has been shown to reduce grey field slug populations.

Click here for further information on slug management.